1. Field of the Invention
The present invention relates to a color cathode ray tube. More specifically, the present invention relates to a color cathode ray tube characterized by a configuration of a mask frame in order to improve image quality, especially color uniformity.
2. Description of Related Art
As shown in FIG. 17, a color cathode ray tube has a glass bulb 13 including a front panel, whose inner surface is provided with a phosphor screen 14, and a funnel. In a neck portion of the glass bulb 13, an electron gun 81 is provided. A shadow mask 1 that is stretched by a mask frame 31 faces the phosphor screen 14. The mask frame 31 has a substantially L-shaped cross-section, and includes a first portion and an inward projecting portion 32; the former stretches the shadow mask 1 and is fixed to the glass bulb 13 and the latter projects toward a tube axis (central axis) side of the glass bulb 13 so as to be substantially in parallel to the shadow mask 1. An inner magnetic shield 2 is fixed to the inward projecting portion 32.
Electron beams 5 corresponding to three colors of R (red), G (green) and B (blue) are emitted from the electron gun 81 and pass through the shadow mask 1 that is located immediately in front of the front panel. Based on the incident angle at the time of this passage, positions at which the electron beams 5 strike the front panel can be restricted. According to these impact positions, therefore, the phosphors of R, G and B separately are applied on the inner surface of the front panel, thereby performing a color selection geometrically, so as to form color images on the phosphor screen 14.
In a regular color cathode ray tube, images are reproduced by an over scan system so that the images are displayed over an entire screen area of the phosphor screen. The amount of this over scan is about 105 to 110% in each of horizontal and vertical directions of the phosphor screen. When the phosphor screen is scanned with such an over scan system, a part of the over-scanning electron beams 5 hits the mask frame 31 supporting the shadow mask 1 and is reflected so as to reach the phosphor screen 14 as shown in FIG. 18, so that a phosphor layer other than that in a predetermined position emits light. This lowers color purity and contrast of the image, thus deteriorating image quality.
In order to prevent the deterioration of the image quality due to this reflected beam, an electron shield 33 conventionally has been formed at a tube-axis-side edge of the inward projecting portion 32 of the mask frame 31 as shown in FIG. 19. Alternatively, as shown in FIG. 20, an electron shield 33 has been provided between the inner magnetic shield 2 and the inward projecting portion 32 of the mask frame 31 so as to protrude beyond the mask frame 31 toward the tube axis side.
However, since the electron shield 33 conventionally has been formed of a magnetic substance, when the cathode ray tube is placed in the presence of a terrestrial magnetism of about 800 A/m (10 Oe), a leakage magnetic field from a front end portion of the electron shield 33 sometimes has caused a phenomenon that the electron beam is subjected to a deflection of its path so as not to strike a desired position of the phosphor layer (mis-landing).
It is an object of the present invention to provide a color cathode ray tube that prevents mis-landing due to a terrestrial magnetism and has no color displacement.
In order to achieve the above-mentioned object, a color cathode ray tube of the present invention includes a mask frame, a shadow mask fixed to the mask frame, an inner magnetic shield supported by the mask frame, and an electron shield provided in the mask frame. At least a part of the electron shield has a smaller anhysteretic magnetic permeability than the shadow mask, the mask frame and the inner magnetic shield when an applied magnetic field is 800 A/m (10 Oe).
Since this configuration increases the magnetic resistance of the electron shield, magnetic flux flowing toward a front end portion of the electron shield can be suppressed, thereby reducing a leakage magnetic field from the front end portion of the electron shield. Thus, it is possible to provide a color cathode ray tube that reduces the mis-landing due to the terrestrial magnetism and has no color displacement.
Also, it is preferable that the electron shield is formed so as to elongate a front end portion on an electron beam side of the mask frame.
Alternatively, it is preferable that the electron shield is formed of a member different from the mask frame so as to protrude beyond a front end portion on an electron beam side of the mask frame.
Also, it is preferable that a part of the electron shield has a region having a smaller anhysteretic magnetic permeability than the other part when the applied magnetic field is 800 A/m (10 Oe).
With this configuration, it is possible to regulate the magnetic flux flowing from the inner magnetic shield via the mask frame toward the front end portion of the electron shield, thereby reducing the leakage magnetic field from the front end portion of the electron shield.
Furthermore, in the above-described color cathode ray tube, it is preferable that the mask frame includes a L-shaped member having a L-shaped cross-section and a reinforcing member connected with the L-shaped member, and a part of the reinforcing member has a region having a smaller anhysteretic magnetic permeability than the other part when the applied magnetic field is 800 A/m (10 Oe).
With this configuration, it is possible to regulate the magnetic flux flowing from the inner magnetic shield toward the reinforcing member of the mask frame, thereby reducing the leakage magnetic field from the reinforcing member of the mask frame.
Moreover, in the above-described color cathode ray tube, it is preferable that, when an electron beam scans a phosphor screen by 100%, a minimum distance between the electron shield and a path of the electron beam is at least 8 mm.
With this configuration, since the electron beam passes through a region where the leakage magnetic field is weak, the mis-landing can be reduced further.